370-373] The Planetesimal Theory 399 
invariable plane which is at right angles to the original axis of rotation. The 
system must remain symmetrical about the plane, to which the axis of rotation 
of the central mass remains perpendicular. 
■ Now the invariable plane of the solar system is practically fixed by the 
orbits of the four outermost planets. Over 99‘9 per cent, of the total momentum 
of the system resides in the orbital motion of these planets, and the orbits of 
Jupiter, Saturn and Neptune all lie within 45' of this plane. But the sun’s 
axis of rotation is not perpendicular to this plane, but makes an angle of 
about 6 ° with the perpendicular. The rotational theory is unable to account 
for the divergence between the invariable plane and the plane of the sun’s 
rotation; the tidal theory explains it very naturally by identifying the present 
plane of the sun’s rotation with that of the original sun, while supposing the 
plane of the orbits of the outer planets to mark the plane of passage of the 
wandering star whose tidal forces tore the planets out of the sun. 
The Planetesimal Theory. 
373 . About 1900, Professors Chamberlin and Moulton of Chicago begun 
to develop a theory of the origin of the solar system, commonly known as 
the Planetesimal Theory*, which has received a great deal of attention in 
America. 
The authors point out that the sun is even now in a state of constant 
eruption, fountains of matter occasionally spouting hundreds of thousands 
of miles above its surface. The matter ejected in this way falls back into the 
sun, but if a near body were exerting tidal forces on the sun, the fountains 
might rise much higher than they do. The planetesimal theory supposes that 
at some past time, while the sun was emitting periodic eruptions, a star came 
so near that the eruptions formed two long arms of matter extending for 
enormous distances out from the sun’s surface. The sun now formed a sort of 
spiral nebula; the matter ejected at the eruptions formed its arms, the 
condensations in the arms representing the ejections of matter at different 
eruptions. The bits of matter, as they were ejected, solidified and formed the 
“planetesimals” after which the theory is named. Those ejected at any one 
eruption aggregated to form a planet; each big eruption had associated with 
it a lot of little eruptions, and the matter ejected at these little eruptions 
formed the satellites of the planets. 
There is a want of precision about the theory which makes it difficult to 
submit to the test of precise mathematical calculation. A dynamical investi 
gation which I made in 1916*}' into the effects of tidal action on a stellar mass 
indicated, however, that the sequence of actual events would be very different 
* See T. C. Chamberlin, Fundamental Problems of Geology (Year Book No. 3 of the Carnegie 
Institution); F. R. Moulton, An Introduction to Astronomy (New York, 1906), pp. 463ff; T. C. 
Chamberlin, The Origin of the Earth (Chicago University Press, 1916). 
f Memoirs R.A.S. lxii. (1916), p. 1.